In the acquisition of seismic data, seismic waves are used to determine the nature, orientation and location of subsurface geological formations. In the late 1950s, Conoco, Inc., pioneered the development of a new type of geophysical prospecting technique, now generally known as “vibroseis” prospecting. Vibroseis prospecting employs a seismic vibrator used to generate a controlled wavetrain that propagates through the earth to be detected by seismic detectors. The vibrator operator selects an energization (sweep) interval, along with a subsequent period during which the vibrator is not energized. Reflected signals are received both during the time when the vibrator is energized and when it is not. Typically, the energization takes the form of a sinusoidal vibration of continuously varying frequency applied to the surface of the earth (or in a body of water) during a sweep period lasting from about two to about 20 seconds or even more.
Various types of sweeps are possible, each employing some sort of amplitude taper applied at the beginning and at the end of the sweep to insure the amplitude of the sweep goes to zero smoothly at its endpoints. The frequency of the sweep may be varied linearly or nonlinearly with time. The standard signal is a linear sweep. A linear sweep is a sinusoidal-type signal having an essentially constant amplitude envelope where the frequency of the signal varies linearly with time, either increases or decreases monotonically within a given frequency range, and yields a constant sweep rate. A non-linear sweep is a sinusoidal-type signal where the frequency varies nonlinearly with time.
Recently, a new type of signal known as a “shaped-sweep” has been developed for use in vibroseis prospecting. Shaped-sweep technology is disclosed in, e.g., U.S. Pat. No. 5,347,494 to Andersen. One benefit of using a shaped-sweep is that the sweep is designed to have an autocorrelation pulse length and an impulse response spectrum that facilitates subsequent data processing activities.
In vibroseis, data that is generated from the vibratory source is correlated with a reference sweep to produce a correlated record. A reference sweep signal is, generally, an ideal signal that the vibrator is programmed to apply. The correlated record resembles a conventional seismic record, as one would receive with an explosive or impulsive seismic source.
It is well known in seismic art that an undesirable byproduct in vibration-generated seismic signals is “side lobe energy.” Side lobes are byproducts of the correlation process and lengthen and complicate the desired correlated seismic wavelet. Visually, side lobe energy appears as small oscillations to either side of the central three lobes of the seismic wavelet. Side lobe energy degrades data quality and affects adversely the ability to estimate and control the seismic wavelet in processing. There is, therefore, a need to generate vibrator correlation data that have a simple wavelet shape with minimal side lobe energy. Such data would reduce seismic signal distortion and enhance seismic resolution.
In attempting to solve the side lobe problem, Rietsch, “Vibroseis Signals with Prescribed Power Spectrum,” Geophysical Prospecting, Vol. 25, pp. 613-620 (1977), developed a relationship between a sweep's instantaneous phase function and its power spectral density for sweeps having a constant amplitude envelope. This relationship is based on the fact that the power spectrum of a sweep is inversely related to the rate of frequency change of the sweep. Rietsch proposed a method for determining an appropriate phase function for a sweep that has a certain predetermined power spectrum, noting that the method could be used to design sweeps with autocorrelation functions that had low side lobes. Sweeps having predefined power spectra could, theoretically, have been designed using this relationship; however, vibrator electronic control systems of that time could not accurately reproduce or follow a user-defined sweep.
Post Rietsch, the advent of new-generation vibrator control instruments based on advanced microprocessor technology allowed for tight control of the vibrator output force (both amplitude and phase). This advancement enabled user-defined sweeps to be more accurately reproduced and followed by the vibrator. For example, Andersen in U.S. Pat. No. 5,347,494, incorporated herein by reference in its entirety, adds to Rietsch's method for producing improved wavelet shapes with minimal side energy by using a feedback loop that compensates for the effects of a non-constant amplitude envelope (taper). Andersen further proposes certain power spectra that produce a substantially three-lobe wavelet. Both Rietsch and Andersen employ methods that relate a sweep's phase function to its power spectrum.
Recently developed cascaded sweep techniques suffer in part from the side lobe problem described above. In these techniques, multiple sweeps (segments) are linked end-to-end, output by the vibrator, and recorded with a single listen period at the end of the cascaded sweep. This significantly reduces the field time required to record the data as compared to conventional methods. The recorded cascaded sweep data are then correlated with a cascaded reference sweep sequence. The correlated data show, due to the cascading of the sweeps, a repetitive (segment) structure. As a result of this structure, side lobes of the first breaks in one segment may extend into a previous segment masking weak reflections and, thus, degrading the data quality.
There is a need in the art for effective and efficient methods to enhance the quality of vibroseis data. Such methods produce wavelets with minimal side lobe energy, solve the side lobe problem in cascaded sweep data, and provide shaped data when cascaded sweeps are used in simultaneous shooting with two or more vibratory sources.